In order to improve a peak rate, achieve compatibility to a LET system and make full use of spectrum resources, current LTE-A (Long Term Evolution-Advanced) introduces a carrier aggregation technology. In a system introduced with the carrier aggregation, an uplink and a downlink of a user equipment (UE) can communicate with base stations (also called BS or eNodeB) through a plurality of cells having a plurality of component carriers (CC). Generally, the plurality of cells where the UE works comprise a primary cell (“Pcell” for short) and a plurality of secondary cells (“Scell” for short). With respect to one UE, if the base station determines that certain cells can share one TA value, these cells can be classified into one TA group.
FIG. 1 schematically illustrates a scenario of a wireless communication system utilizing the above carrier aggregation technology. As shown in FIG. 1, the base station supports two frequency bands (or cells) F1 and F2, wherein F1 cell provides large coverage scope and is assumed as a primary cell whereas F2 cell provides small coverage scope and assumed as a secondary cell. The coverage scope of the F2 cell can be expanded by a frequency selective repeater.
In this scenario, when the UE is located in a region jointly covered by the F1 cell and F2 cell and being close to the base station, it can be considered that the F1 and F2 cells use the same TA value, so the F1 cell and F2 cell belong to the same TA group. When the UE moves far away from the base station, and is located in coverage scope of the repeater, the F1 cell and the F2 cell can be aggregated. However, since a distance of the UE from the base station is distinct from a distance of the UE from the repeater, the uplink TA values of the F1 cell and F2 cell are different so that the F1 cell and F2 cell will belong to different TA groups.
In the foregoing exemplary scenario, due to constant movement of the UE or other causes, the current secondary cell of the UE might belong to different TA groups so that the UE uses different new TA values, which inevitably involves the issue of a TA group change. How to detect such a TA group change and subsequently support such a TA group change is a problem that needs to be solved in the art.